Public debt has difference between internal and external. Most of Singapore's (and Japan's) public debt is internal, i.e. the government owe debt obligation to their own people. That's why SgD and JpY are so strong in forex, they're actually net creditors in the international debt market. It's unlike US where foreigners hold most of their debt and even their public join in the borrowing. That's why USD keeps sliding down.
While SGD remains strong but SG people keep sliding down because the terms of repayment keep changing?

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